Isotope‐Sensitive Degenerate [1,3]‐Hydrogen Migration versus Competitive Enol–Keto Tautomerization

Abstract

The question of intramolecular energy distribution in polyatomic molecules or, more precisely, whether reacting molecules display ergodic or non-ergodic behavior, is of central importance in reaction dynamics.[1] For ionic gas-phase systems, the unimolecular behavior of the enol and keto forms of ionized acetone, 1 and 2, respectively, constitutes one of the best studied systems. [2,3] Detailed 2 H-and 13 C-labeling experiments, energetic measurements, analysis of kinetic energy release distributions as well as electronic-structure calculations of the potential-energy surface with the inclusion of trajectory studies reveal the following (Scheme 1): Direct dissociation of 1 to generate the hydroxyvinyl cation 4 does not occur; rather, irreversible isomerization to a highly excited, short-lived acetone ion 2 takes place, which decomposes to the acylium ion 3 concomitant with loss of a CH 3 radical at a time-scale of 5 10 À13 s. From 2, the newly formed methyl group is eliminated preferentially as compared to the one which is already present in 1, and the kinetic energy release associated with elimination of this process is smaller than that for the loss of the newly generated methyl group. All these findings clearly point to a non-statistical (i.e., "non-ergodic") behavior of the chemically activated acetone ion 2. Inspired by the detailed mechanistic information which can be achieved from kinetic analysis of multiple labeling data, [4] here we report a re-investigation of the dissociation of 1 for a wide set of isotopologues, generated via dissociative electron ionization of the corresponding labeled 2-hexanones in a McLafferty reaction. [5,6] To our surprise, however, it turned out that the unimolecular dissociation of ionized acetone and its enol form offers additional mechanistic puzzles which have not been recognized before. [2,3] The experimentally obtained data for the unimolecular dissociation of the enol ions 1-1 h (Table 1) can be accounted for by a rate-determining, irreversible enol-keto tautomerization 1 ! 2 followed by a rapid dissociation of the latter in which the two methyl groups do not have sufficient time to completely equilibrate their originally different energy content. As a consequence, both CÀCH 3 bonds are cleaved with different rates. This view has been supported by numerous computational studies. [1b,2c,d,f,h,i,j] Further, the analysis of the data for 1-1 h implies that a degenerate [1,3]-hydrogen migration, by which the methyl and the methylene groups of 1 equilibrate, cannot energetically compete with the tautomerization 1 ! 2. This is in line with exploratory, [a] Dr.

Gas‐phase acylium ion transfer reactions mediated by a proton shuttle mechanism

“…Model calculations at the MP2/6‐31+G(d,p) level for formic acid reveal that this [1,3] H‐migration is characterized by a high energy barrier of 51 kcal mol −1 , a value within 1 kcal mol −1 of that previously calculated at the G2 level50. This high energy barrier is similar to barriers encountered for tautomerization processes in simple gas‐phase ionic systems through [1,3]‐H migration in ionic systems51–53. At room temperature, and in the absence of some kind of activation process for the ions, the tautomerization shown in Scheme would be highly unfavorable under thermal conditions.…”

“…Two possibilities can be considered for generating the hydroxy‐protonated acid: Intramolecular [1,3]‐H migration as shown in Scheme .Model calculations at the MP2/6‐31+G(d,p) level for formic acid reveal that this [1,3] H‐migration is characterized by a high energy barrier of 51 kcal mol −1 , a value within 1 kcal mol −1 of that previously calculated at the G2 level50. This high energy barrier is similar to barriers encountered for tautomerization processes in simple gas‐phase ionic systems through [1,3]‐H migration in ionic systems51–53. At room temperature, and in the absence of some kind of activation process for the ions, the tautomerization shown in Scheme would be highly unfavorable under thermal conditions. A water mediated proton‐shuttle followed by displacement.Several experimental and theoretical examples have been illustrated in recent years, where tautomerization processes of gas‐phase ions can be catalyzed by a proton shuttle mechanism54–58.…”

QM/MM Protocol for Direct Molecular Dynamics of Chemical Reactions in Solution: The Water-Accelerated Diels–Alder Reaction

“…An increasing number of studies point to the importance of nonstatistical dynamics in organic reactions. − Experimental rate or product ratios in these reactions are poorly described by statistical theories (transition state theory or RRKM theory) but can be understood in terms of nonstatistical dynamical effects revealed by classical trajectory calculations. One area of concern is that most reactions are carried out in solution, but most trajectory calculations either have been applied to gas-phase reactions or have used a dynamical model without explicit solvent for reactions that occur in solution.…”

Nonstatistical Dynamics in Unlikely Places: [1,5] Hydrogen Migration in Chemically Activated Cyclopentadiene

“…Accumulating evidence from laboratories around the world, studying an ever-widening range of reaction types, points to an important role for nonstatistical dynamical effects in the thermal reactions of polyatomic molecules. − When such effects occur, the results can be disruptive to the conventional models of reaction mechanisms. For example, reactive intermediates can behave in ways that depend on how they were prepared, they can form products in ratios that are not predictable from the barriers on the standard free energy surface (in extreme cases even favoring the pathway that has the highest barrier), and they can choose exit channels in a manner that shows oscillatory time dependence .…”